吊打面试官:从 szieof 对齐到Linux kernel 宏函数container_of

#define offsetof(type, member) /* 编译器实现 */
[1] https://en.cppreference.com/w/cpp/types/offsetof.html

//判断是否 pod 或者标准 布局
https://en.cppreference.com/w/cpp/types/is_standard_layout.html
https://en.cppreference.com/w/cpp/types/is_pod.html
#include <type_traits>
 
struct A { int m; };
static_assert(std::is_standard_layout_v<A>  true);
 
class B: public A { int m; };
static_assert(std::is_standard_layout_v<B>  false);
 
struct C { virtual void foo(); };
static_assert(std::is_standard_layout_v<C> == false);
 
int main()
{}

#include <cstddef>
#include <iostream>

struct S 
{
    char m0;     // 偏移量 0
    double m1;   // 偏移量 8（可能有填充）
    short m2;    // 偏移量 16
    char m3;     // 偏移量 18
//private:
   //  private: int z; // warning: 'S' is a non-standard-layout type
};

int main() {
    std::cout << "offset of char m0 = " << offsetof(S, m0) << '\n'
              << "offset of double m1 = " << offsetof(S, m1) << '\n'
              << "offset of short m2 = " << offsetof(S, m2) << '\n'
              << "offset of char m3 = " << offsetof(S, m3) << '\n';
}

offset of char m0 = 0offset of double m1 = 8offset of short m2 = 16offset of char m3 = 18

struct A {
    char  c;   // 1 字节
    int   i;   // 4 字节
    char  d;   // 1 字节
};

偏移地址      内容       说明
----------------------------------
0            c          char 占 1 字节
1            padding    填充 3 字节，使下一个成员 i 按 4 字节对齐
2            padding
3            padding
----------------------------------
4~7          i          int 占 4 字节
----------------------------------
8            d          char 占 1 字节
9            padding    填充，为保证整体结构体大小是 4 的倍数
10           padding
11           padding
----------------------------------
sizeof(A) = 12 字节

struct B {
    double d;  // 8 字节
    char   c;  // 1 字节
    short  s;  // 2 字节
};

代码：github.com:watchpoints/master-cpp.git

#include <stdio.h>

#include <stddef.h>

struct B 
{

	double d;

	char c;

	short s;

};



int main()
{

	printf("Sizeof(struct B) = %zu\n", sizeof(struct B));

	printf("Offset of d: %zu\n", offsetof(struct B, d));

	printf("Offset of c: %zu\n", offsetof(struct B, c));

	printf("Offset of s: %zu\n", offsetof(struct B, s));

	return 0;

}

/*

Sizeof(struct B) = 16

Offset of d: 0

Offset of c: 8

Offset of s: 10

*/

/**  
 * container_of - cast a member of a structure out to the containing structure  
 * @ptr:    the pointer to the member.  
 * @type:   the type of the container struct this is embedded in.  
 * @member: the name of the member within the struct.  
 *  
 */
#define container_of(ptr, type, member) ({          \  
    const typeof( ((type *)0)->member ) *__mptr = (ptr); \  
    (type *)( (char *)__mptr - offsetof(type,member) );})

https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/linux/container_of.h?h=v5.16.3

#include <linux/container_of.h>
//定义宿主结构体（其中可以包含任意个成员，且成员可以是任意排列顺序）
struct demo {
    char *name;
    struct list_head list; // 链表节点嵌入其中
};

//从list获取name
void demo_list_to_name(struct list_head *lst)
{
    struct demo *dm = container_of(lst, strcut demo, list);
    printk(KERN_INFO "%s\n", dm->name);
}

//通过给子函数传递demo结构体中的list的指针，找到demo中的name变量
void demo_main(void)
{
    struct demo dm;
    dm.name = "Demo";
    demo_list_to_name(&dm->list);
}